in Geometry, any part of the circumfer- ence of a circle or curved line, lying from one point to another, by which the quantity of the whole circle or line, or some other thing sought after, may be ga- thered. See Bridge and Center.

a concave or hollowed piece of building, constructed in such a manner that the several stones of which it is composed keep one another in their places. The terms arch and vault properly differ only in this, that the arch expresses a narrower, and the vault a broader piece of the same kind. The principal dif- ference in the form of arches is, that some are circu- lar, and others elliptical; the former having a larger or smaller part of a circle, the other an ellipsis. What are called flat arches, are those frequently used for doors and windows, the upper and under edges of which are straight and parallel, and the ends and joints. Arch joints all pointing toward a centre. The space between two piers of a bridge is called an arch, because usually arched over.

Triumphal ARCHES are magnificent entries into cities, erected to adorn a triumph, and perpetuate the memory of the action. The arches of Titus and Constantine make at this time a great figure among the ruins of old Rome.

in composition, signifies chief, or of the first class: as archangel, archbishop, &c.

ARCHÆUS, or ARCHEUS. See ARCHEUS.